The present invention relates to a plasma processing apparatus and a plasma processing method; and, in particular, the invention relates to an apparatus for etching an insulation film, such as a silicon oxide film of a wafer, using a plasma. The invention relates to a plasma etching apparatus and a plasma etching method having a plasma generation source which is capable of effecting a minute processing of an etching pattern, and is further able to maintain a stable etching characteristic during a long period of operation.
One example of a conventional plasma processing apparatuses is an oxide film plasma etching apparatus, and the techniques used thereby and the problems inherent in this apparatus are known. As a conventional plasma source for use in an oxide film plasma etching apparatus, a type which is used most widely is a narrow electrode type high frequency plasma generation apparatus, which is comprised of a pair of opposed electrodes.
Of the known systems of the narrow electrode type of high frequency plasma generation apparatus, there is a system in which a high frequency from 13.56 MHZ to a several 10 MHZ is applied to one electrode and a wafer a high frequency bias of about 1 MHZ is applied separately to the electrode on which a wafer is mounted, and there is another system in which a high frequency is applied to the pair of electrodes.
In this narrow electrode type of plasma source etching apparatus, since the distance between the electrodes is narrow, for example, from 20 mm to 30 mm, it is known as a narrow electrode type plasma source and a parallel flat plate type plasma source. Further, in the narrow electrode type plasma source, it is difficult to generate a plasma in a region where the pressure is low, however, by the addition of a magnetic field, an apparatus is obtained in which a lowering of the discharge pressure can be achieved.
In addition to the above-stated narrow electrode type of apparatus, other plasma etching apparatuses have been known. These apparatuses include a plasma etching apparatus having an induction type plasma source in which an induction coil is used and another plasma etching apparatus in which a plasma etching microwave is introduced. In these apparatuses having an induction type etching source and a microwave type plasma source, it is possible to generate and maintain the plasma under a low pressure; and, since the plasma density is high, such a plasma source is known as a low pressure and a high density plasma source.
In silicon oxide film etching, as an etching gas, a mixture gas, in which argon (Ar), a gas including carbon (C) and fluorine (F), such as C4F8, and a gas including hydrogen (H), such as CHF3, are mixed, is used; and, further, another mixture gas, in which oxygen (O2) and carbon monoxide (CO) and hydrogen (H2) etc. are added to the above-stated mixture gas, is used. These gases are dissociated by the plasma and are dissolved to form CF3, CF2, CF, and F. The amount and the ratio of this gas molecule species exerts a large influence on the etching characteristic of the silicon oxide film (hereinafter, it will be referred to merely as an “oxide film”).
In particular, in the case of a high density plasma source, since the electron temperature in the plasma is high, plasma dissociation progresses, and the plasma comes to have many fluorine gas molecules F. Further, as the ionization progresses, the ratio of neutral gas molecule species (radicals) becomes low. For these reasons, in oxide film etching with a high electron temperature and a high density plasma, since the amount of CFx (CF3, CF2, CF) which adheres to a silicon surface, which is a foundation of the oxide film, is lowered, there are problems in that the etching-speed of the silicon (Si) is large and the selection ratio is small.
As means for solving the above stated problems, a method for increasing the CFx radical amount in the plasma has been known, in which the temperature of the wall face of the etching chamber is raised to about 200° C., in an effort to discharge the deposition film which has adhered to the wall face by reducing the adhesion of the deposition film to the wall face of the etching chamber. As a result, in an apparatus in which a high density plasma is used, to obtain the desired selection ratio, a high temperature performance of the wall face of the etching chamber becomes indispensable.
An oxide film etching apparatus described in Japanese application patent laid-open publication No. Hei 7-183283 is an example of an apparatus in which a wall face of an etching chamber is formed to have a high temperature performance.
As a countermeasure for obtaining the high selection ratio in addition to the above technique, there is a known method in which the electron temperature in the plasma is lowered and plasma dissociation is restrained. More specifically, in this method the plasma application is carried out intermittently, and so this method is called a pulse plasma method.
As another one example of obtaining a high selection ratio, there is a method in which materials for consuming fluorine (F) are installed in an etching 25, chamber in advance. In Japanese application patent laid-open publication No. Hei 9-283494, such a method is described, in which a side wall of an etching chamber is constituted by silicon (Si), and a heating means for heating the side wall and a bias application means are provided, so that the fluorine (F) in the plasma is consumed.
In oxide film etching in which narrow electrode type of plasma generation is used, in correspondence with the fine patterning in which a device pattern size is less than 0.25 μm, it is necessary to make the scattering of the ion incident angle at a portion to be subjected to the etching extremely small. Since the scattering of the ion incident angle causes an abnormality of the etching shape and a decrease in the number of ions reaching the bottom of a deep hole, problems are caused including a lowering of the etching speed and a premature stopping of the etching in the formation of holes. This scattering of the ion incident angle is caused by the incident angle distribution having a spread angle because the ions collide with radicals in the plasma.
To solve the above-stated problems, it is effective to decrease the number of collisions between ions and radicals; more particularly, it is necessary to lower the pressure. As a result, in the narrow electrode type of plasma generation apparatus, because it is difficult to carry out the plasma discharge under low pressure conditions, even under a low pressure sufficient to generate a plasma, it is proposed that the frequency of the plasma generation source be made high and that a magnetic field be applied.
Further, in the narrow electrode type of plasma source in which the distance between the electrodes is narrow, in a case where a low pressure is used, since the average free path distance of the gas molecules becomes long, the collision frequency of the gas molecules is decreased, and, in place of this, the collision between the gas molecules and the electrode becomes dominant.
This is not a preferable condition, since, in the etching apparatus, according to the collision of the gas molecules in the plasma, it is necessary to control the maintenance and the reaction of the plasma; and, as a result, in order to accommodate a lower pressurization, it is necessary to provide a large electrode interval.
When the electrode interval is wide, the surface area of the side wall in the etching chamber becomes large. Here, the surface of the etching chamber is the surface which is subjected to the plasma, and the surface does not include a surface of the top plate (ceiling), a surface of the floor, and a surface of the electrode (the wafer).
Until now, in the narrow electrode type plasma source, from the aspect of the plasma and a wafer, since the side wall area is narrow, the deposition and the gas discharge at the side wall have almost no influence on the etching characteristic; however, in the narrow electrode type plasma apparatus in which a low pressurization is used, it is necessary to take a new countermeasure.
Further, to accommodate a large diameter wafer, it is necessary to make the gas pressure distribution across the wafer face and the reaction product distribution uniform; and, for this purpose, it is necessary to provide a wide electrode interval, and so the area ratio of the side wall becomes more and more important.
The influence of the affects of the reaction products which adhere to the side wall on the etching characteristic is discussed above, however, when the etching is continued over a long period of time, a change of the influence becomes a problem. For example, by repeatedly carrying out etching operation, the temperature of the side wall will rise gradually. When the temperature of the side wall has risen sufficiently, the characteristic of the adhesion and adsorption of the reaction products on the side wall is changed, and, as a result, the etching characteristic fluctuates.
Further, in a case where the amount of the deposition film on the side wall accompanying the etching is increased gradually, in accordance with the dependence on the amount of the deposition film, it is possible to change the desorption and adsorption characteristic of the reaction products at the side wall surface.
A phenomenon in which the etching characteristic is influenced by the time lapse change stated above is known particularly in the case of oxide film etching. As a result, the temperature change of the side wall in the oxide film etching apparatus represents an important problem.
In particular, in a high electron temperature and high density plasma source, it is necessary to establish a high side wall temperature. In the case of a high side wall temperature, even the side wall temperature fluctuates a little, and so the adsorption and desorption characteristic of the deposition film is changed largely. For these reasons, it is necessary to restrain the side wall temperature fluctuation to a small range, and a high accuracy temperature adjustment, such as 200° C.±2° C. needs to be carried out.
As stated above, in any of the plasma sources, to satisfy the requirement for oxide film etching, namely for obtaining a high etching speed, while attaining a high selection ratio, low micro loading, and the passing-through of a deep hole, there still remain problems to be solved.
The important problem in an oxide film etching apparatus involves the dissociation of the gas molecules as the plasma is being formed under the most suitable conditions for the etching of the oxide film. To address this problem, a new plasma generation source producing a high density plasma under a low electron temperature has been proposed. For example, Japanese application patent laid-open publication No. Hei 8-300039, disclosed a UHF type ECR apparatus having a plasma excitation frequency in the UHF band from 300 MHZ to 1 GHz. The electron temperature of the plasma which is excited in the frequency band in the above stated range is low, for example, from 0.25 eV to 1 eV, and the plasma dissociation of C4F8 is at a level suitable to oxide film etching. Further, since it is an ECR (Electron Cyclotron Resonance) system, even under a low pressure, it is possible to generate a high density plasma.
As stated above, for achieving fine patterning on a wafer of large diameter, it is necessary to make the electron temperature low and to prevent an excessive dissociation of the etching gas, and further to make the plasma density high. Further, it is necessary to make the plasma density, the gas pressure and the reaction product distribution on the wafer uniform; and, as a result, it is necessary to provide an apparatus in which the oxide film etching characteristic is not changed over a long period of operation.